GEOGRAPHICAL DISTRIBUTION. 97 



or sixty fathoms being found in them. This fact indicates that 

 subsidence was probably most extensive to the' south, and per- 

 haps also most rapid. The sinking of the Chagos Bank still fur- 

 ther south, in nearly the same line, may therefore have some 

 connection with the subsidence of the Maldives. 



In view of the facts which have been presented, it appears 

 that each coral atoll once formed a fringing reef around a high 

 island. The fringing reef, as the island subsided, became a bar- 

 rier reef, which continued its growth while the land was slowly 

 disappearing. The area of waters within finally contained the 

 last, sinking peak: another period, and this had gone — the island 

 had sunk, leaving only the barrier at the surface and an islet or 

 two of coral in the enclosed lagoon. Thus the coral wreath 

 thrown around the lofty island to beautify and protect, becomes 

 afterwards its monument, and the only record of its past exis- 

 tence. The Pan mo tu Archipelago is a vast island cemetery, 

 where each atoll marks the site of a buried island. The whole 

 Pacific is scattered over with these simple memorials, and they 

 are the brightest spots in that desert of waters. 



5. Geographical Distribution of Coral Reefs and Islands. 



The distribution of coral reefs over the globe depends on the 

 following circumstances, arising from the habitudes of polyps 

 already explained : 



1. The temperature of the ocean. 



2. The character of coasts as regards (a) the depth of water, 

 — (b) the nature of the shores, — (c) the presence of streams. 



3. Liability to exposure to destructive agents, such as volcanic 

 heat. 



It has been stated that reef-growing corals* will flourish in the 

 hottest seas of the equator, and over the ocean wherever the 

 winter temperature is not below 6(3° P. The isocheimal line of 

 this temperature therefore forms the boundary line of the coral- 

 reef seas. 



This line traverses the oceans between the parallels 26° and 

 30°, or in general near 28°. But in the vicinity of the conti- 

 nents it undergoes remarkable flexures, from the influence of 

 oceanic currents, the polar currents bending it towards the equa- 

 tor, while the tropical cause a divergence. From a comparison 

 of the thermometrical observations of various voyagers with 

 those of the Expedition, I have been enabled to draw this coral 

 boundary with a considerable degree of accuracy ; and it is laid 



* There are corals in colder seas ; we here refer to those species that form reefs. 



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